The invention relates generally to ultrasound imaging systems, and, more particularly, to a medical diagnostic configurable ultrasound imaging system that includes a wide dynamic range continuous wave (CW) Doppler receiver for monitoring blood flow.
Ultrasound imaging systems have been available for quite some time and are commonly used in nondestructive testing and medical applications. Medical ultrasound imaging allows the internal structure of the human body to be viewed non-invasively in real time by transmitting ultrasound energy into the body through a transducer, and receiving ultrasound echoes reflecting from tissue and blood in the body. Because of the complex environment of the human body as a reflector of ultrasound energy, the ultrasound system contains highly sensitive processing circuitry. The ultrasound system should be able to distinguish between wanted and unwanted signals that occur simultaneously over a wide dynamic range.
Conventional ultrasound imaging systems typically are capable of many different modes of operation. For example, black and white ultrasound imaging can be used to image various internal structures of the human body that are presented to the user as a two dimensional picture of the structures. Doppler imaging is a mode of operation that allows the movement of fluid, such as blood within a vein or artery, to be imaged and displayed as a waveform that plots the velocity of the blood flow over time. The velocity of the blood flow can also be presented as an audio signal. The hardware that performs the signal processing for these imaging modes consists of both analog and digital circuitry. Analog circuitry is used where the signal-to-noise ratio, or the dynamic range, of the signals cannot tolerate the limitations imposed by digital signal processing circuitry, specifically the process of digitizing the analog signal. Once the analog processing has tailored the signals to an adequate dynamic range, the signals can be digitized. Because of the diverse nature of the various imaging modes in an ultrasound system, multiple processing paths exist to process signals having different dynamic range requirements.
Doppler imaging can be performed using either pulse wave (PW) or continuous wave (CW) techniques. PW Doppler typically involves generating and transmitting a periodic pulse wave through a transducer at a certain operating frequency that is directed to a particular location having blood flow. The signal reflecting from the moving blood is shifted in frequency by an amount proportional to the velocity of the blood flow (the xe2x80x9cDoppler Effectxe2x80x9d). This frequency is received by the transducer, the receiver typically being collocated with the transmitter. The direction of the blood flow can also be determined by the xe2x80x9csign,xe2x80x9d or relative polarity, of the frequency difference between the transmitted and received signals. The velocity of the blood flow that is sought to be measured determines the rate at which the pulse wave is transmitted. The pulses should be sent at a rate sufficient to analyze the velocity of the blood flow. Unfortunately, due to limitations in the rate at which the pulse wave can be transmitted, PW Doppler is limited in its ability to measure very high blood flow velocities. The rate limitations exist because the echoes generated by the transmitter must propagate into the body and are reflected back to the receiver before another pulse can be sent, based on the depth of the blood flow being interrogated.
CW Doppler, on the other hand, typically involves generating and transmitting a constant continuous wave signal toward the area to be imaged at a particular transducer operating frequency. The signal is continuously reflected by the blood flow, and is received by a receiver located in close proximity to the transmitter. The receiver distinguishes between the transmitted signal and the received signal by determining if there is a frequency shift between the transmitted and received signals. The movement of the blood causes this frequency shift, where the frequency shift is proportional to the velocity of the blood, and the direction of the blood flow is dependent on whether the frequency of the received signal is greater or less than the frequency of the transmitted signal. Because the signal is transmitted continuously, CW Doppler can detect significantly higher frequency shifts than PW Doppler since there is no inherent sampling rate limitation in CW mode.
Unfortunately, compared to PW Doppler ultrasound, it is more difficult for CW Doppler ultrasound to distinguish between the transmitted signal and the reflected signal that represent blood flow for two reasons. First, because the transmit signal is continuous and relatively high in amplitude, it generates interference in the receiver. Second, the echoes reflected from non-moving tissue do not contain a frequency shift, and are considered unwanted signals relative to the signals reflecting from moving blood. These sources of interference, typically called xe2x80x9cclutter,xe2x80x9d are troublesome because the signal that represents blood flow is in general very small in magnitude when compared to the transmit signal or the echoes from stationary tissue. The wide dynamic range difference between the transmit and stationary echo signals and the reflected signal (if the reflected signal indicates blood flow) in a CW Doppler system requires that a separate analog processing path be implemented in an ultrasound imaging system that includes CW Doppler functionality.
The separate analog processing path for a CW Doppler receiver typically consists of cascaded stages of mixers and filters that detect the frequency shift of the received signal, and that filter unwanted clutter signals and unwanted high frequency components from the received signal. To support a variety of transducers, the hardware typically includes a number of programmable filters that are tuned to the operating frequencies of the available transducers. Also, the mixing stages are typically programmable in their frequencies of operation. The hardware implementation of these programmable stages typically includes costly switches and precision components.
Unfortunately, this hardware design dictates that the design be single-ended (where each signal is referenced to ground (zero (0) volts). Single-ended operation results in additional noise that may be generated by a power supply, or that appears at signal ground, being added to the received signal. This further reduces the signal-to-noise ratio and dynamic range of the received signal. Such a single-ended mode of operation can limit the performance of the receiver because common-mode noise (such as power supply noise and interference) directly affects the performance of the CW Doppler receiver.
In addition, to maximize the dynamic range of the CW Doppler processing circuitry, CW Doppler receivers typically include numerous programmable high-pass filters. These high-pass filters are commonly referred to as xe2x80x9cwall filters,xe2x80x9d or xe2x80x9cclutter filtersxe2x80x9d because they attempt to remove the undesired tissue echo signals from the overall received signal. Because of the limited dynamic range of typical CW ultrasound receivers, a variety of clutter filters is necessary to handle the range of clutter frequencies generated by the tissue surrounding the blood flow. These clutter filters are costly and prone to generating xe2x80x9cmirroring artifactsxe2x80x9d if they are not properly adjusted. Mirroring artifacts manifest themselves as the inability to determine the direction of the blood flow. The processing erroneously presents flow in both directions when mirroring occurs. In order to provide a variety of programmable clutter filters, the hardware requires costly switches and precision components, which forces the design to operate single-ended.
Further, conventional CW Doppler receivers have a limited dynamic range due to the limited dynamic range of the analog-to-digital (A/D) converters used in the signal processing path. Because of the limited dynamic range of the A/D converter, the clutter filtering that precedes the A/D converter must be much more complex, so that the signal that feeds the A/D contains as little clutter content as possible.
Therefore, it would be desirable to maximize the efficiency of the analog processing path used in a CW Doppler receiver and improve the dynamic range and sensitivity of a CW Doppler receiver.
The invention is a Doppler receiver that has a wide dynamic range and operates in a fully differential mode. In one embodiment, the invention is a wide dynamic range continuous wave (CW) Doppler receiver, comprising a differential fixed frequency output mixer configured to receive a radio frequency (RF) input signal and supply a differential signal output; a fixed frequency differential bandpass filter coupled to the fixed frequency differential output mixer, configured to provide a differential fixed frequency output; an in-phase mixer and a quadrature-phase mixer, each configured to receive a differential signal output of the differential fixed frequency bandpass filter; at least one in-phase differential low-pass filter configured to receive the output of the in-phase mixer and supply a filtered in-phase signal; at least one quadrature-phase differential low-pass filter configured to receive the output of the quadrature-phase mixer and supply a filtered quadrature-phase signal; a fixed differential high-pass clutter filter; and an analog-to-digital converter configured to receive the filtered in-phase signal and the filtered quadrature-phase signal and supply a corresponding in-phase digital output and a corresponding quadrature-phase digital output, the output of the analog-to-digital converter supplied to a memory element.
Related methods of operation and computer readable media are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.